Batch thermal processing continues to be used for several stages of fabrication of silicon integrated circuits. One low temperature thermal process deposits a layer of silicon nitride by chemical vapor deposition, typically using chlorosilane and ammonia as the precursor gases at temperatures in the range of about 700° C. Other, high-temperature processes include oxidation, annealing, silicidation, and other processes typically using higher temperatures, for example above 1000° C. or even 1350° C.
For large-scale commercial production, vertical furnaces and vertically arranged wafer towers supporting a large number of wafers in the furnace are typically used, often in a configuration illustrated in the schematic cross-sectional view of FIG. 1. A furnace 10 includes a thermally insulating heater canister 12 supporting a resistive heating coil 14 powered by an unillustrated electrical power supply. A bell jar 16, typically composed of quartz, includes a roof and fits within the heating coil 14. An open-ended liner 18 fits within the bell jar 16. A support tower 20 sits on a pedestal 22 and during processing the pedestal 22 and support tower 20 are generally surrounded by the liner 18. It includes vertically arranged slots for holding multiple horizontally disposed wafers to be thermally processed in batch mode. A gas injector 24 is principally disposed between the liner 18 has an outlet on its upper end for injecting processing gas within the liner 18. An unillustrated vacuum pump removes the processing gas through the bottom of the bell jar 16. The heater canister 12, bell jar 16, and liner 18 may be raised vertically to allow wafers to be transferred to and from the tower 20, although in some configurations these elements remain stationary while an elevator raises and lowers the pedestal 22 and loaded tower 20 into and out of the bottom of furnace 10.
The bell jar 18, which is closed on its upper end, tends to cause the furnace 10 to have a generally uniformly hot temperature in the middle and upper portions of the furnace. This is referred to as the hot zone in which the temperature is controlled for the optimized thermal process. However, the open bottom end of the bell jar 18 and the mechanical support of the pedestal 22 causes the lower end of the furnace to have a lower temperature, often low enough that the thermal process such as chemical vapor deposition is not effective. The hot zone may exclude some of the lower slots of the tower 20.
Conventionally in low-temperature applications, the tower, liner, and injectors have been composed of quartz or fused silica. However, quartz towers and injectors are being supplanted by silicon towers and injectors. One configuration of a silicon tower available from Integrated Materials, Inc. of Sunnyvale, California is illustrated in the orthographic view of FIG. 2. It includes silicon bases. 30, 32 bonded to three or four silicon legs 34 having slots formed therein to support multiple wafers 38. The shape and length of the fingers between the slots varies with the application and process temperature. The fabrication of such a tower is described by Boyle et al. in U.S. Pat. No. 6,455,395. Silicon injectors are also available from Integrated Materials, as disclosed by. Zehavi et al. in U.S. patent application Ser no. 11/177,808, filed July 8, 2005. Silicon liners have been proposed by Boyle et al. in U.S. patent application Ser No. 09/860,392, filed May 18, 2001 and published as U.S. Patent Application Publication 2002/170486
The height of the tower can be modified according to the height of the furnace and may include slots for over 100 wafers. Such a large number of wafers has prompted the use of thermal buffer wafers and dummy wafers to assure that the production wafers are subjected to a uniform thermal environment. Both the top and the bottom of the stack of wafers in the tower during thermal process are subject to thermal end effects. Particularly, the bottom wafers are heated to a significantly lower temperature and the temperature may be low enough that the nitride CVD process or other thermal process is inactive. Accordingly, thermal buffer wafers rather than the substantially monocrystalline silicon production wafers are placed in the topmost and bottommost slots to thermally buffer the ends of the stack and provide a more uniform temperature distribution for the production wafers placed in between. The thermal buffer wafers also act to scavenge impurities from the furnace ambient that tend to be more populous in the top and bottom of the furnace. It is not uncommon to use up to six or twelve thermal buffer wafers on each end. The buffer wafers may be reused for multiple cycles, but current baffle wafers are typically limited to no more than four or five cycles.
Silicon production wafers are often processed in batches of about 25 wafers, corresponding to the capacity of carrying cassettes transporting them between fabrication tools. The large number of wafer slots allows multiple batches to be simultaneously processed. However, there are situations when less than the maximum number of batches need thermal processing. In these situations it is common to nonetheless fully populate the tower by inserting dummy wafers in the empty slots.
Thermal buffer wafers and dummy wafers will be jointly referred to as baffle wafers.
In the past in conjunction with quartz towers, the baffle wafers were typically composed of quartz (fused silica), which are inexpensive and have the further advantage of being opaque to infrared radiation to thereby reduce the end effects of radiation greater than 4.5 μm (the quartz window) bathing the tower. However, just like quartz towers, quartz buffer and dummy wafers have been recognized to contribute to the generation of particles to a degree unsatisfactory for the fabrication of advanced devices. The use of production type of monocrystalline silicon wafers as baffle wafers have not been completely successful. They have been observed to fracture easily in repeated use. Further in nitride deposition process, the silicon nitride is deposited on the baffle wafers to greater thicknesses in multiple uses and has been observed flake off, again creating a particle problems. As a result, in advanced production monocrystalline silicon baffle wafers are limited to a lifetime of only a few cycles before they are discarded or refurbished.
Silicon carbide baffle wafers have also been used, particularly at higher temperatures. However, silicon carbide wafers are expensive and are also subject to effects arising from the differential coefficient of thermal expansion between a silicon carbide wafer and a silicon tower.
Accordingly, less expensive baffle wafers are desired which nonetheless provide superior performance including ruggedness and ability to have great thickness of nitride and other material deposited thereon without flaking.